KB08
PACKAGE SPECIFICATION
HSL 2013
1 SUMMARY
To sort an array of numbers into descending order maintaining an index array to preserve a record of the
original order.
The ’Quicksort’ algorithm is used, see, C.A.R. Hoare, ’Quicksort’, Computer Journal, April 1962.
ATTRIBUTES — Version: 1.0.0. Types: Real (single, double), Integer. Original date: April 1980. Origin:
C.Birch*, Harwell.
2 HOW TO USE THE PACKAGE
2.1 The argument list and calling sequence
Sorting single precision numbers
CALL KB08A(ARRAY,N,INDEX)
Sorting double precision numbers
CALL KB08AD(ARRAY,N,INDEX)
Sorting integer numbers
CALL KB08AI(ARRAY,N,INDEX)
ARRAY is an array containing the numbers to be sorted and the user must put these in ARRAY(I), I=1,N. On return
from the subroutine they will have been sorted into descending order. ARRAY should be of the Fortran type
corresponding to the name of the sorting subroutine being used.
N
is an INTEGER variable and must be set by the user to the number of numbers in the array.
INDEX is an INTEGER array of length at least N, which will be set by the subroutine to the original (unsorted) order of
the numbers in ARRAY, so that reference may be made to both orderings of the array. On return from the
subroutine the I th element of the index array, J=INDEX(I), gives the position in the original ordering of the
number now in ARRAY(I), i.e. the sorting process has moved the number which was originally in ARRAY(J) to
ARRAY(I).
3 GENERAL INFORMATION
Use of Common: none.
Workspace: private integer workspace of length 100, which limits the size of ARRAY to 2 50 ≈ 10 15 .
Other subroutines: none.
Input/Output: prints an error message if n < 1.
Restrictions: n ≥ 1.
4 METHOD
The ’Quicksort’ method is used, see C.A.R. Hoare, ’Quicksort’, Computer Journal, April 1962.
All use is subject to licence.
http://www.hsl.rl.ac.uk/
1
KB08 v 1.0.0
Documentation date: 19th March 2013
KB08
HSL 2013
5 EXAMPLE OF USE
This example program uses KB08AD to sort the N numbers in the array ARRAY into descending order while
preserving the original ordering in the array INDEX.
PROGRAM MAIN
INTEGER N, I
PARAMETER ( N = 10 )
INTEGER INDEX( N )
DOUBLE PRECISION ARRAY( N )
DATA ARRAY / 1.0D0, 5.0D0, 7.0D0, 0.0D0, 4.0D0,
*
6.0D0, 2.0D0, 3.0D0, 9.0D0, 8.0D0 /
CALL KB08AD( ARRAY, N, INDEX )
WRITE( 6, "( ' reordered array = ', /,
*
' index array ', /, ( I6, F6.2 ) )" )
* ( INDEX( I ), ARRAY( I ), I = 1, N )
STOP
END
This produces the following output:
reordered array =
index array
9 9.00
10 8.00
3 7.00
6 6.00
2 5.00
5 4.00
8 3.00
7 2.00
1 1.00
4 0.00
All use is subject to licence.
http://www.hsl.rl.ac.uk/
2
KB08 v 1.0.0
Documentation date: 19th March 2013